Apparatus for eliminating interelectrode leakage in a photomultiplier type exposure control system by providing a compensating voltage



R. PAULUS PROVIDING A COMPENSATING VOLTAGE Filed Oct. 23, 1965 Aug. 27, l

56 7 6 9 O 18 I9 20 T A 21 w 7 32 l V 38 g 7 O 1 12 r 1 14 2a I 39 32a 29 'z'zzzzzazz F ig.l

INVENTOR.

RUDOLF PAULUS United States Patent 3,399,304 APPARATUS FOR ELIMINATING INTERELEC- TRODE LEAKAGE IN A PHOTOMULTIPLIER TYPE EXPOSURE CONTROL SYSTEM BY PRO- VIDING A COMPENSATING VOLTAGE Rudolf Paulus, Munich, Germany, assignor to Agfa- Gevaert Aktiengesellschaft, Leverkusen, Germany Filed Oct. 23, 1965, Ser. No. 502,931

Claims priority, application Germany, Nov. 27, 1964, A 47,714 11 Claims. (Cl. 250207) ABSTRACT OF THE DISCLOSURE An exposure regulating circuit for photographic apparatus. A secondary electron multiplier having a photocathode, an anode and -a plurality of dynodes is positioned so as to be exposed to the light impinging upon the photographic apparatus. This same light impinges also upon the photocathode and is amplified so as to provide a photocurrent at the anode corresponding to the intensity of the light. An alternating voltage is applied to the secondary electron multiplier through a transformer. The primary winding of the transformer is energized through an AC. current. A plurality of taps on the secondary winding of the transformer leads to the photocathode and the dynodes of the secondary electron multiplier. An impedance is connected to the anode and has a voltage drop across it in accordance with the photocurrent resulting at the anode from the light impinging upon the cathode. When the voltage drop across the impedance attains a predetermined value, a signal is transmitted which actuates the exposure circuit of the photographic apparatus. The noise effects resulting from the AC, voltage and stray capacitance of the electron multiplier are compensated by applying a compensating voltage to the impedance and thereby provides substantially accurate exposure control.

The present invention relates to an exposure regulating circuit with compensation for noise voltage. More particularly, the invention relates to an exposure regulating circuit for photographic copying apparatus or the like With compensation for noise voltage.

In an exposure regulating circuit for photographic copying apparatus or the like, a secondary electron multiplier comprising a plurality of dynodes for electron multiplication is energized by AC or pulsating DC and has an impedance connected to an electrode thereof. The dynodes of the electron multiplier are connected to taps of a voltage divider to which the AC or pulsating DC is applied. The voltage drop across the impedance corresponds to the intesity of light impinging on the electron multiplier and is an indication of the quantity of light impinging on the copying material, Such an exposure regulating circuit is well known. Often, a capacitive reactance is utilized as the impedance. The capacitor or capacitive reactance is charged to a predetermined voltage by the anode current of the electron multiplier and when the capacitor reaches such predetermined voltage it terminates the exposure.

The secondary electron multipliers utilized are very sensitive and are thus expensive. They react to extraneous light such as, for example, that caused by temporary illumination of the dark room. In order to permit the use of secondary electron multipliers of lesser sensitivity, or to utilize the very sensitive electron multipliers for copying material of higher sensitivity, it is necessary to keep the capacitance of the capacitive reactance as small as possible. A reduction of the capacitance of the capacitive reactance rapidly reduces the accuracy of the control operation.

"ice

Although the dynodes and photocathode of the secondary electron multiplier are separated by insulating resistors of sufficiently high resistance value, there is considerable scatter between the dynodes and the photocathode and between thedynodes and the cathode and the anode. Since the energizing current for the secondary electron multiplier is generally not a pure sine wave, but is usually a sine wave with clipped peaks, the voltage in the electron multiplier is susceptible to extraneous quadrature voltage. Thus, very steep voltage pulses are transmitted to the anode via the scatter capacities. If the capacitance of the capacitive reactance is small, the scatter currents or noise are superimposed on the photocurrent provided at the anode and are increasingly magnified. It is practically impossible to reduce the scatter capacities to the required extent.

The principal object of the present invention is to provide a new and improved exposure regulating circuit with compensation for noise voltage.

An object of the present invention is to provide an exposure regulating circuit which compensates for noise voltage with a simple structure utilizing a minimum of components.

Another object of the present invention is to provide an exposure regulating circuit which compensates for secondary electron multiplier noise with efiiciency, effectiveness, and reliability.

In accordance with the present invention, an exposure regulating circuit for controlling an exposure control circuit comprises a secondary electron multiplier having a photocathode, an anode, and a plurality of dynodes positioned between the photocathode and the anode for amplifying light impinging upon the photocathode and providing at the anode a photocurrent corresponding to the light. A transformer has a primary winding to which an energizing voltage is applied and a secondary winding having a plurality of taps, each of the photocathode and dynodes of the secondary electron multiplier being connected to a corresponding one of the taps of the secondary winding of the transformer. A capacitive reactance is connected to the anode of the secondary electron multiplier and has a voltage produced thereacross in accordance with the photocurrent provided at the anode of the secondary electron multiplier, the photocurrent including noise produced by the secondary electron multiplier. The capacitive reactance is connected to an exposure control circuit in a manner whereby when the voltage across the capacitive reactance reaches a predetermined level the voltage controls the exposure control circuit to terminate an exposure. A compensating voltage is produced by a portion of the secondary Winding of the transformer which is connected to a point at ground potential at its junction with the remainder of the secondary winding, and a variable coupling capacitor is connected between the winding potential and the capacitive reactance to apply the compensating voltage to the capacitive reactance to compensate for noise in the photocurrent.

In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawing, wherein:

FIG. 1 is a circuit diagram of an embodiment of the exposure regulating circuit of the present invention with compensation for noise voltage; and

FIG. 2 is a simplified schematic diagram of the embodiment of FIG. 1.

In the figures, the same components are indicated by the same reference numerals.

In FIG. 1, the exposure regulating circuit comprises a transformer 2 having a primary winding 1 which is energized by an AC voltage source 38. The transformer 2 has a secondary winding 3 having a plurality of taps 4.

5, 6, 7, 8, 9, 10, 11, 12, 13, 14 and 15. A secondary electron multiplier 17 comprises a photocathode 16 connected to the tap 4 of the secondary winding 3, an anode 28 and a plurality of dynodes 18, 19, 20, 21, 22, 23, 24, and 26 positioned between the photocathode and the anode. The dynodes 18, 19, 20, 21, 22, 23, 24, 25 and 26 are connected to the taps 5, 6, 7, 8, 9, 10, 11, 12, and 13, respectively of the secondary winding 3 of the transformer 2.

In accord-ance with the present invention, the inter mediate tap 14 of the secondary winding 3 is connected to a point at ground potential and the winding portion of said secondary winding between the taps 14 and 15 functions to produce a compensating voltage for the noise superimposed on the photocurrent provided at the anode 28.

The anode 28 of the secondary electron multiplier 17 is connected to an impedance comprising a capacitive reactance provided by a capacitor 29. The capacitor 29 is charged to a predetermined voltage by the anode 28 of the electron multiplier 17 and when the capacitor voltage reaches the predetermined level, it is utilized to terminate an exposure by controlling the operation of a tube 33 having a grid 34 which is connected to said capacitor and to said anode via the common point 39. The tube 33 is a component of a switch unit 30 which comprises a DC voltage source 31 and a relay 32 connected in series circuit arrangement with said tube. The tube 33 may comprise, for example, a cold-cathode thyratron. The relay 32 operates the switch 32a of photographic apparatus 41 which is exposed to the light impinging on the photocathode 16 of the electron multiplier 17.

Further in accordance with the present invention, a variable coupling capacitor 27 is connected between the tap 15 of the compensating voltage winding portion 14, 15 and the common point 39 in the connection between the anode 28 and the capacitor 29. The variable coupling capacitor 27 is also connected to the grid 34 of the tube 33. The capacitor 29 is connected to a point at ground potential.

The potential on the photocathode 16 and the dynodes 18 to 26 of the electron multiplier 17 is negative and becomes less negative as the distance from said photocathode increases due to the taps on the secondary winding 3 of the transformer 2 Light impinging on the photo cathode 16 produces electrons, which are multiplied by the dynodes 18 to 26 and appear as a photocurrent at the anode 28. The photocurrent charges the capacitor 29 until it reaches the predetermined potential at which said capacitor discharges and switches the tube 33 to its conductive condition. When the tube 33 becomes conductive, it closes the switch circuit 30 and a current flows through the energizing winding of the relay 32 to operate or energize said relay. When the relay 32 is energized it operates a switch (not shown) to terminate the exposure of the photographic copying apparatus or the like (not shown).

The secondary electron multiplier 17 produces scatter currents or noise which is superimposed upon the photocurrent provided at the anode 28 thereof. If this noise is not compensated, it causes the capacitor 29 to reach its predetermined potential prematurely so that said capacitor discharges prematurely and prematurely switches the tube 33 to its conductive condition thereby terminating the exposure prematurely. In order to compensate for the noise to prevent premature termination of the exposure, in accordance with the present invention, a compensating voltage is produced by the compensating voltage winding portion 14, 15 of the secondary winding 3 of the transformer 2, and is added via the coupling capacitor 27 to the charging voltage applied to the capacitor 29 to compensate for the noise and thereby negate the effect of the noise and prevent premature discharge of said capacitor 29 and thus prevent premature termination of the exposure.

The tap 14 is connected to a point at ground potential in accordance with the present invention, so that the compensating voltage produced by the compensating voltage winding portion 14, 15 of the secondary winding 3 is equal and opposite to the noise superimposed on the photocurrent provided at the anode 28. The tap 14 is approximately half way between the taps 14 and 15 of the secondary winding 3 of the transformer 2. The compensating voltage is thus the same as the voltage applied to the cathode and dynodes of the electron multiplier 17 but opposite in polarity, so that it cancels or negates the noise in the charging voltage of the capacitor 29. In accordance with the present invention, noise compensation is provided with a simple structure and a minimum of components, that is, with the compensating voltage winding portion 14, 15 and the variable coupling capacitor 27.

The secondary electron multiplier noise may be compensated by the circuit of the present invention when the capacitor 27 is connected to the dynode 26 instead of the anode 28 of said secondary electron multiplier. Furthermore, the secondary electron multiplier noise may be compensated by the circuit of the present invention it a component other than a capacitor is utilized as the impedance 29, Such a component may comprise, for example, an inductive reactance, a diode or a resistor. The voltage drop across such impedance component would be amplified and utilized to control the exposure time.

The simplified diagram of FIG. 2 aids in explaining the operation of the circuit of the present invention. FIG. 2 shows only the voltage source 38, the transformer 2 and its primary winding 1, its secondary winding 3 and its taps 4, 14 and 15, the capacitor 29 and the variable coupling capacitor 27 of FIG. 1. The capacitor 35 represents the sum of the scatter capacities between the cathode 16, dynodes 18 to 26 and anode 28 of the secondary electron multiplier 17 (FIG. 1).

A voltage V1 is produced between the tap 4 and the tap 14, which is connected to a point at ground potential, and an equal but opposite polarity voltage V2 is produced between the tap 14 and the tap 15, If the circuit of FIG. 2 is considered to be an AC differential bridge, it is readily seen that since the common point 39 in the connection between the anode 28 of the secondary electron multiplier 17 and the capacitors 29 and 27 is connected to a point at ground potential via the capacitor 29, the following relationship occurs:

wherein C27 and C35 are the capacitances of the capacitors 27 and 35, respectively. The variable coupling capacitor 27 may thus be varied in capacitance to the point where the AC of the anode 28 of the electron multiplier 17 (FIG. 1) is virtually connected to a point at ground potential.

The AC differential bridger does not comprise pure capacitances, but rather resistances and capacitances, since there is insulation resistance in each capacitor and resistance in the circuit connections. If the time constants of the branches of the bridge are different, the adjustment becomes frequency-dependent, that is, steep voltage pulses are not completely compensated. Such steep voltage pulses are, however, reduced to the point where the charging voltage of the capacitor 29 is reduced sufiiciently to prevent premature discharge of said capacitor and thereby prevent premature termination of the exposure.

While the invention has been described by means of a specific example and in a specific embodiment, I do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

What I claim is:

1. An exposure regulating circuit for controlling an exposure control circuit comprising a secondary electron multiplier having a photocathode,

an anode and a plurality of dynodes positioned between said photocathode and said anode for amplifying light impinging upon said photocathode and providing at said anode a photocurrent corresponding to the magnitude of said light;

transformer means having a primary winding and a secondary 'winding having a plurality of taps, each of the photocathode and dynodes of said secondary electron multiplier being connected to a corresponding one of the taps of the secondary winding of said transformer means;

means for applying a varying energizing voltage to the primary winding of said transformer means and thereby apply varying voltage levels to the photocathode and dynodes of said secondary electron multiplier;

impedance means connected to one of the anode and a dynode of said secondary electron multiplier and having a voltage produced thereacross in accordance with the photocurrent provided at the anode of said secondary electron multiplier, said photocurrent including noise resulting from the stray capacitance of said secondary electron multiplier;

connecting means connecting said impedance means to an exposure control circuit in a manner whereby when the voltage across said impedance means reaches a predetermined level said voltage controls said exposure control circuit to terminate an exposure; and

compensating means for producing a compensating voltage and applying said compensating voltage to said impedance means to compensate for the noise in said photocurrent.

2. An exposure regulating circuit as claimed in claim 1, wherein said impedance means is connected to the anode of said secondary electron multiplier.

3. An exposure regulating circuit as claimed in claim 2, wherein said impedance means comprises a capacitive reactance.

4. An exposure regulating circuit as claimed in claim 1, wherein said compensating means comprises a portion of the secondary winding of said transformer means, said portion of said secondary winding being connected to a point at ground potential at its junction with the remainder of said secondary winding, said winding portion producing said compensating voltage.

5. An exposure regulating circuit as claimed in claim 4, wherein said compensating means further comprises a coupling capacitor connected between said Winding portion and said impedance means.

6. An exposure regulating circuit as claimed in claim 4, wherein said winding portion is approximately half said secondary winding.

7. An exposure regulating circuit as claimed in claim 5, wherein said coupling capacitor is a variable coupling capacitor.

8. An exposure regulating circuit as claimed in claim 3, wherein said compensating means comprises a portion of the secondary winding of said transformer means, said portion of said secondary winding being connected to a point at ground potential at its junction with the remainder of said secondary winding, said winding portion producing said compensating voltage.

9. An exposure regulating circuit as claimed in claim 8, wherein said compensating means further comprises a coupling capacitor connected between said winding portion and said impedance means.

10. An exposure regulating circuit as claimed in claim 8, wherein said winding portion is approximately half said secondary winding.

11. An exposure regulating circuit as claimed in claim 8, wherein said coupling capacitor is a variable coupling capacitor.

References Cited UNITED STATES PATENTS 2,913,585 11/1959 Rodman 250-207 RALPH G. NILSON, Primary Examiner.

M. ABRAMSON, Assistant Examiner. 

